Road Trip: Time vs Gas mileage?

Not that I'm for speeding, as I actually only go over 5-10mph over the limit, but I do ride with people who think the black numbers are just there as an option... on some tech channel (not sure if net or TV, been a few months now) there was a device you bought that also synced with your smart phone (if you have one). It detected both Radar and Laser. The syncing part to the phone was to mark that area as a trap on a road map, and you could also see where others (if others happen to be using the same set up/data base) had been tagged with radar/laser.

As to the question in the OP, my car can tell be how many miles until I "run out", so I just point and go.